Centrifugal pellet dryer apparatus

ABSTRACT

A centrifugal pellet dryer apparatus, lifter and rotor wherein the lifters can have a surface configured to deflect pellets inwardly and the rotor can have lifters attached in an arrangement designed to increase pellet impacts with lifters by providing a higher concentration of lifters on the rotor in a first region which initially engages a slurry of pellets and water. The inwardly curved lifters tend to direct the pellets inwards toward other lifters and away from dewatering screens and the higher concentration of lifters on the lower region of the rotor results in added pellet impacts with lifters. The centrifugal pellet dryer can have an outer housing, a base portion with a water discharge outlet and a top portion having a pellet discharge port and an exhaust port. Within the housing can be one or more generally cylindrical foraminous members disposed around the rotor. One or more separator plates can be provided between the foraminous members to divide the pellet dryer into sections. The rotor with lifters affixed rotates within the foraminous members to direct pellets upwardly to the pellet discharge port which can have a counter-flow air inlet.

RELATED APPLICATIONS

This application is a Divisional Application of co-pending U.S. patentapplication Ser. No. 09/685,282, filed Oct. 10, 2000.

BACKGROUND

The invention relates generally to centrifugal pellet dryers utilized todry plastic pellets which have been cut from strands of plastic by apelletizer, and more particularly, to a centrifugal pellet dryerapparatus including improved rotor and lifters which can provideenhanced dewatering capabilities. The invention also relates toadditional improvements in the centrifugal pellet dryer apparatus whichcan further enhance dewatering capabilities.

Generally, a water slurry of plastic pellets is introduced into a pelletdryer for separation of the pellets from the water. The dry pellets canthen be conveyed to a shipping container or to a location for furtherprocessing. Centrifugal pellet dryers are well known in the art. Inparticular, U.S. Pat. No. 5,611,150, to Yore, Jr., issued Mar. 18, 1997,which is hereby incorporated herein by reference, discloses a“Centrifugal Pellet Dryer” wherein a slurry of pellets and water isintroduced upwardly through the bottom of the dryer, into a hollowregion in the rotor and out through ports in the rotor. In this manner,the slurry is directed radially outwards from generally the center ofthe rotor. The rotor has lifters, i.e. blades, which carry the pelletsand water upwardly through the dryer as the water is forced outwardly bycentrifugal force through a cylindrical screen that surrounds the rotor.The dewatered pellets are discharged through an exit port near the topof the cylindrical screen while the separated water is drained throughthe bottom of the dryer housing. Prior to the centrifugal pellet dryerdescribed in the '150 patent, centrifugal pellet dryers introduced theslurry of pellets and water through an entry port in the side of thepellet dryer. The slurry was injected inwardly towards the rotor and thelifter affixed to the outside of the rotor drove the slurry outwardsagainst the screen as the rotor was rotated. In that configuration, therotor breaks the stream of the slurry where it is introduced through theside of the pellet dryer, causing a large portion of the pellets to bedeposited on the portion of the screen nearest the slurry entry port. Incontrast, the pellet dryer disclosed in the '150 patent describesintroducing the slurry of pellets and water through the center of therotor and radially outward into the space between the rotor and the meshmember. By introducing the pellets and water through the center of therotor, the slurry of pellets and water is radially, and more evenly,distributed by the rotor lifters about the interior of the pellet dryer.Thus, pellet impacts are not concentrated at any single area of thescreen, in contrast to the conventional pellet dryers which injectedslurry through a side port where most of the initial dewatering is donein the first quadrant adjacent to the slurry inlet, which loads up thescreen and limits its effectiveness in the initial dewatering stage.

Introducing the slurry through the rotor also minimizes radial bearingloads that are present in the prior-pellet dryers since the lifterblades in those pellet dryers must break the stream of water and pelletsin a single location which is off-center from the axis of the rotor. Thefeed of the slurry through center of the rotor also enhances thecentrifugal force of the rotor to throw the water radially through thescreen to thereby enhance the initial dewatering stage of the dryer.

Recently issued U.S. Pat. No. 5,987,769, to Ackerman, et al., issuedNov. 23, 1999, discloses an alternative manner of introducing a slurryof pellets and water generally into the center of the pellet dryer.However, the slurry is not injected radially outwards through the rotoras in the '150 patent. Rather, the slurry is injected axially upwardsonto a lower face of the rotor. Lifters, or blades, are provided on alower face of the rotor which throw the slurry radially outwards againstthe cylindrical screen. A disadvantage of this pellet dryer can be thatthe lower end of the rotor is not supported. This is because the slurryis directed upwards against the lower face of the rotor. Thus, the rotoris supported only at the top of the dryer, which can be disadvantageoussince it is desirable to support a rotating member like the rotor atboth ends to maintain axial alignment. Supporting the rotor at only oneend can create problems with balance and vibration when the rotor isrotated, especially when rotated under a load as when slurry is beingpumped by the rotor through the pellet dryer. Accordingly, it cangenerally be desirable to support the rotor at both ends to improveefficiency and longevity of the dryer.

Conventionally, it was generally believed that dewatering was bestaccomplished by impacting the pellets against the dewatering screen toremove the water. However, it has been discovered that dewatering canbest be achieved by instead limiting the number of impacts between thepellets and the dewatering screen and increasing the number of impactsbetween the pellets and the lifters. Residual water on the screen canactually be reacquired by pellets through repeated impacts with thescreen. The slurry of pellets and water can typically be introduced intothe pellet dryer while the pellets are still hot. The internal heat ofthe pellets actually assists in the drying process. An additional reasonwhy pellet impacts against the dewatering screen can be disadvantageousis that the screen acts like a “grater,” shaving or breaking off piecesof the pellets during impacts. These pieces of pellets, commonly called“fines,” can cause other problems with the operation of the pellet dryerand with disposal of the water removed from the slurry. Consequently,lifters which reduce the number of impacts with the dewatering screenscan improve the drying efficiency and reduce fines. Lifters can bedesigned to reduce the number of impacts with the dewatering screens bycreating the lifters with an inwardly curved surface which tends todirect the pellets away from the dewatering screen and back in towardsthe rotor surface and other lifters. This additionally results inincreased impacts between the lifters and the pellets.

In prior pellet dryers, generally flat lifter blades are attached to thesurface of the rotor in a helix configuration at a 45 degree angle. The45 degree angle is what “lifts” the pellets upwards through the pelletdryer. The lifters are generally flat in that there is no curvatureapart from the helix curvature imparted as a result of attachment to thecylindrical rotor. The flat lifters direct the pellets out into thedewatering screen. The lifters are also conventionally attached to thesurface of the rotor in an evenly distributed manner, in that there arethe same amount of lifters in each row along the entire length of therotor. Prior art pellet dryers typically use 5-6 lifters evenly spacedradially around the circumference of the rotor. The lifters are alsoconventionally aligned horizontally in rows and vertically in columns.

Accordingly, there is a need for a lifter, rotor and pellet dryerapparatus which provides improved dewatering capabilities throughincreased pellet impacts with the lifters and reduced pellet impactswith the dewatering screen.

SUMMARY

According to the invention, a centrifugal pellet dryer apparatus, rotorand lifter is provided wherein the lifters can have a front surfaceconfigured to deflect pellets inwardly toward the rotor surface andother lifters, and the rotor can have lifters attached in aconfiguration designed to increase pellet impacts with lifter blades aswell as providing different regions of lifters wherein different numbersof lifters can be provided in the different regions of lifters along thelength of the rotor. The specially configured surface of the lifterstend to control the pellet path, keeping the pellets in the lifterenvelope and away from the dewatering screens. The configuration of thelifters on the rotor, generally a higher concentration of lifters on thelower region of the rotor can further create increased pellet impactswith the lifters.

The centrifugal pellet dryer can have an outer housing with a waterremoval port formed in the outer housing and one or more mesh membersdisposed vertically within the outer housing. The mesh member can beformed of material that permits passage of water while blocking thepassage of pellets therethrough. A vertically disposed rotor withlifters affixed thereto is journaled for rotation coaxially within thedewatering screen to direct pellets upwardly to a pellet discharge portin an upper portion of the dryer. The rotor can have a hollow interiorportion coaxial with the rotor and a plurality of radial passagesextending between the hollow interior portion and the outer surface ofthe rotor. A slurry inlet is provided adjacent the bottom of the rotorin communication with the hollow interior portion of the rotor. Slurryintroduced through the slurry inlet is conducted through the hollowinterior portion of the rotor and through the radial passages whichdirect the slurry into a space between the outer surface of the rotorand the dewatering screen.

The lifters, also called blades, are provided on the outer surface ofthe rotor in a helix configuration and are angled approximately 45degrees which “lifts” the pellets upwardly toward the pellet dischargeport. To reduce pellet impacts with the screen, and to increase pelletimpacts with the lifters, the surface of the lifters can be configuredto direct pellets inwardly towards the rotor and other lifters, and awayfrom the dewatering screen. The number or rows, the number of lifters ineach row provided in different regions along the length of the rotor,and spacing/alignment of the lifters affixed to the outer surface of therotor can be designed to further increase the number of impacts betweenthe lifters and the pellets.

The centrifugal pellet dryer can also be internally chamberized intomultiple sections for example: a slurry inlet and initial dewateringsection; a secondary dewatering and drying section; and a pelletdischarge section. In this three section embodiment, each section canemploy a dewatering screen, which may extend for all or only a part ofeach section.

An exhaust port through the top of the housing can extend into the upperand middle sections for creating a negative pressure, i.e., a vacuum, inthose sections to enhance a vertical counter flow of air through theupper and middle sections to pull moisture down from the dischargesection. A separator plate can be provided between each section tosomewhat seal each section to enhance the effect of the negativepressure to inhibit the upward movement of moisture into the upperdrying and discharge sections. A hole can be provided through theseparator plates in the upper and middle sections for creating anegative pressure in those sections via the exhaust port. A water sealcan be created at the water outlet ports to create an air tight waterseal, and the slurry inlet tube can be elbow-shaped to provide a watertrap preventing air from entering through the slurry inlet. The waterseal and water trap can inhibit air from entering the lower section andmoisture laden air from rising up through the dryer.

The centrifugal pellet dryer can also be designed so that the differentsections are modular and can thus be interchangeable, as well as forconvenience of assembly and disassembly of the dryer. The modularsections can also be designed such that the size of the pellet dryer,with respect to the length, can be varied for a given diameter pelletdryer simply by varying the number of secondary dewatering sectionsprovided between the slurry inlet and pellet discharge sections. Thefirst and last sections would remain the same for a given diameterpellet dryer. However, the length of the rotor would have to bedifferent to accommodate the changing length resulting from the additionor subtraction of secondary dewatering sections.

Portions of the rotor can be provided with different lifterconfigurations associated with the different sections of the dryer. Forexample, the lower portion of the rotor which is disposed within theslurry entry and initial dewatering section can be provided with agreater number, and different configuration, of lifters designed toinitially separate most of the water out of the slurry. The middleportion of the rotor associated with the secondary dewatering and dryingsections can have fewer lifters and can be differently configured. Sincemost of the water has already been removed in the first section, onlythe minimum number blades required to maintain an upward travel of thepellets toward the discharge portion are needed in the middle sections.

An air inlet, preferably tangential, communicating the atmosphere withthe pellet discharge port can be provided to permit air to be drawn fromthe atmosphere as the pellets are discharged. The counter-flow air thuscreated can eliminate the creation of a disadvantageous flow of airdrawn through the pellet discharge port which could otherwise occur.

Other features can include a lower bearing for the rotor which runs inwater. Failure of the lower rotor bearing due to contamination by waterfrom the slurry is a known problem with conventional centrifugal pelletdryers. Thus, a water-cooled bearing, typically made from a type ofplastic, can eliminate such bearing failure problems. For ease ofcleaning and maintenance, the outer housing and the screen can besegmented such that they can be along a side and removed from around therotor. Additionally, the motor can be located directly. above the rotorto drive the rotor directly, thus eliminating belts and sheaves thatproduce radial loads on the rotor bearings. This can result in longerlasting bearings as well as eliminating belts and sheaves which wear outand must be regularly replaced.

Other details, objects, and advantages of the invention will becomeapparent from the following detailed description and the accompanyingdrawings figures of certain embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a side view of a centrifugal pellet dryer according to theinvention.

FIG. 2 is a top view of the centrifugal pellet dryer shown in FIG. 1.

FIG. 3 is a side view of the centrifugal pellet dryer similar to thatshown in FIG. 1 having the outer cover removed to show the dewateringscreens.

FIG. 4 is a section view taken along line IV—IV in FIG. 2.

FIG. 5 is a section taken along the line V—V in FIG. 4.

FIG. 6 is a perspective view of an embodiment of a rotor manifold.

FIG. 7 is an embodiment of a feed auger.

FIG. 8 is an embodiment of a base member.

FIG. 9 is a cross section taken along line IX—IX in FIG. 2.

FIG. 10 is a section view taken along the line X—X in FIG. 9.

FIG. 11 is a perspective view of a pellet discharge member.

FIG. 12 is a side view of an embodiment of a rotor.

FIGS. 13 and 14 are perspective views of embodiments of structuralmembers of a pellet dryer.

FIGS. 15-18 are perspective views of a presently preferred embodiment ofa lifter having an inwardly deflecting surface.

FIG. 19 is a side view of a rotor blank illustrating a presentlypreferred lifter attachment pattern.

FIGS. 20A-20H are section views corresponding to sections A—A throughH—H in FIG. 19.

FIG. 21 is a section view showing a bearing which runs in water.

FIG. 22 is a top view of the water bearing in FIG. 21.

FIG. 23 is a side cross-section view of an embodiment of the lowerportion of a pellet dryer.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

Referring to the drawing figures, wherein like reference numbers referto similar parts through the several views, a presently preferredembodiment of a centrifugal pellet dryer 30 is shown in FIGS. 1 and 2.The pellet dryer 30 can have a bottom portion 33, an intermediateportion 36 and a top portion 39. The bottom portion can include a slurryinlet 42 for introducing a slurry of pellets and water into thecentrifugal pellet dryer 30 and also water outlet ports 45 fordischarging water from the slurry. The top portion 39 can include apellet discharge outlet 48 and can have an exhaust port 51 for applyinga vacuum within the pellet dryer 30. The centrifugal pellet dryer 30 isshown in FIG. 3 with the housing 54 removed. The housing 54 canpreferably be an outer cover which is openable longitudinally so that itmay be opened and removed from around the intermediate portion 36, whichpermits easy cleaning and 360 degrees of access. This also provides easyaccess to and removal of dewatering the screens 80, 83, 86. Thedewatering screens 80, 83, 86, or other suitable foraminous member, canhave openings sized to retain pellets while permitting water separatedfrom the pellets which is thrown off during the drying process to bedrained from the pellet dryer 30. Although the preferred embodimentsutilize multiple separate dewatering screens, it is to be understoodthat a single screen could also be used. FIG. 3 illustrates anembodiment of the centrifugal pellet dryer 30 that can have threesections 63, 66, 69 each divided by a separator plate 71, 74, 77. Eachsection can include a cylindrical dewatering screen 80, 83, 86.Alternatively, a separator plate need not be provided between everydewatering screen 80, 83, 86. Similarly, one of the single dewateringscreens illustrated could be replaced by two shorter dewatering screenswith no separator plate therebetween. In other words, although separatorplates can be used to section the pellet dryer into chambers byproviding them between dewatering screens, each individual dewateringscreen need not define each chamber. Rather, the separator plates definethe chambers, and one or more dewatering screens may be disposed in eachchamber.

Each dewatering screen 80, 83, 86 may extend the entire length of asection 63, 66, 69, such as shown in the first two sections 66, 69.However, a dewatering screen may also have a non-foraminous portion 81such that the screened portion may extend only partially the length of asection, as in the upper section 63. In the top section 63, it can bepreferable that the non-screened top portion 81 be provided because ithas been determined that, this can aid in creating a downward air flowin the gap 87 outside the dewatering screens 80, 83, 86. It can alsoinhibit the upward travel of moisture up through the gap 116 between therotor 89 and the dewatering screens 80, 83, 86 from the lower regions ofthe pellet dryer 30 near the slurry inlet 42. Openings can be providedin one or more of the separator plates 71, 74, 77, shown best in FIGS.13 and 14, to permit a vacuum applied via the vacuum/exhaust port 51 tobe applied in one or all of the lower sections 66, 69. Each dewateringscreen 80, 83, 86, like the outer housing 54, can be opened along alongitudinal length thereof for convenient removal from around a rotor,as shown in FIGS. 4-12, which is positioned for rotation within thecentrifugal pellet dryer 30. The separator plates 71, 74, 77 can beutilized, and in combination with providing a vacuum via exhaust port51, to inhibit the upward movement of moisture from the. lower section63 of the pellet dryer 30 into the upper sections 66, 69. It has beendetermined that the separator plate 71, 74, 77 help to create a negativepressure in the sections which works with the negative pressure, i.e.,vacuum, which can be created via exhaust port 51. An opening through oneor all of the separator ports (shown best in FIGS. 13-16) can beprovided so that the vacuum created via exhaust port 51 can be createdwithin one or more of the sections 63, 66, 69.

Additionally, the water outlet ports 45 enable creation of an airtightseal and the slurry inlet 42 creates a water trap, inhibiting the entryof air. The slurry inlet 42 and the water outlet ports 45 are sealed toprevent air from entering the lower section and moisture laden air fromraising up through the upper portions 63, 66 of the pellet dryer 30.

Additional details of the pellet dryer 30 are illustrated in FIG. 4, across sectional view taken along line IV—IV in FIG. 2, wherein thestructure inside the outer cover 54 and the dewatering screens 80, 83,86 is shown. As can be seen, the centrifugal pellet dryer 30 can includea rotor 89 positioned therein for rotation generally in the center ofthe pellet dryer 30, within the dewatering screen 80, 83, 86. The rotor89 can have a number of lifters 92, also called blades, affixed to theouter surface 95 thereof for both separating the pellets from the waterslurry and carrying the pellets upwardly within the pellet dryer 30 andout through the discharge port 48. The rotor 89 can preferably besupported at both upper and lower ends thereof. The upper end of therotor 89 includes a shaft 98 which can be connected to a drive shaft ofthe motor 57 positioned on the top portion 39 for rotating the rotor 89directly. Connecting the rotor 89 directly to the output shaft of themotor 57 can be desirable since it eliminates the use of belts andsheaves that produce a radial load on bearings, and which requireregular maintenance and would thus increase the cost and downtime of thepellet dryer 30.

Further details of the lower portion 33 of the centrifugal pellet dryer30 are shown in FIGS. 5 through 8. Particularly, the bottom portion 33can include a base member 101, a feed auger 104, and a rotor manifold107 are shown. The rotor manifold 107 can be connected between the lowerend of the rotor 89 and the base member 101. As best shown in the crosssection view in FIG. 4, the rotor manifold 107 has a hollow interiorportion 110 which is in fluid communication with the slurry inlet port42. The rotor manifold 107 can also have a number of radially extendingpassages 113 connecting the hollow interior portion 110 of the rotormanifold 107 to a gap 116 between the outer surface 95 of the rotor 89and the surrounding dewatering screens 80, 83, 86, shown best in FIGS. 5and 6. The slurry of water and pellets can be conducted by the feedauger 104 from the slurry inlet port 42 in the base member 101 into thehollow interior portion 110 of the rotor manifold 89. As the rotor 89 isbeing rotated by the. motor 57, the slurry is conducted through theradially extending passages 113 and directed radially outwards into thegap 116 between the rotor 89 and the dewatering screen 80, 83, 86. Asshown best in FIGS. 6 through 8, the rotor manifold 107 has a lowerportion 119 which is adapted to be received in a receptacle portion 122formed in the base member 101. The auger feed 104 can be positioned inthe hollow interior portion 122 and have a shaft portion 125 connectedto the lower end of the rotor 89 for rotation therewith. The feed auger104 can be utilized to generate an initial slurry lift—to begin the flowof the slurry upwardly through the hollow interior portion 110 of therotor manifold 107 and out through the radially extending passages 113until the rotation of the rotor 89 and the lift blades 92 begin to pumpthe slurry through the pellet dryer 30. The slurry lift provided by theauger feed 104 permits the slurry level to be flush with the bottom ofthe pellet dryer 30. Other components at the base portion 33 of thepellet dryer can include a base plate positioned over the base member101, shown best in FIG. 5, which can be connected to the base member 101using fasteners connect via a number of fastener openings 130 in thebase plate 128 which are positioned to align with fastener openings 131in the base member 101. One manner of assembling an embodiment of amulti-sectioned centrifugal pellet dryer will be explained more below,in connection with FIGS. 13 and 14.

Referring now to FIGS. 9 through 11, another cross sectional view of anembodiment of the centrifugal pellet dryer 30 is illustrated showingmore details of the upper portion 39, and particularly a pelletdischarge member 134. In the section view illustrated in FIG. 10, thepellet discharge member 134 can be seen along with the exhaust port 51.A perspective view of an embodiment of a pellet discharge member 134 isshown in more detail in FIG. 11. As can be seen in FIGS. 9 and 10, atangential air inlet 137 can be provided in communication with thepellet discharge port 48. The tangential air inlet 137 facilitates thedischarge of dried pellets by providing for counter flow air. Withoutthe tangential air inlet 137, the flow of exiting pellets would beslowed because air would be drawn in through the pellet discharge port48. The pellet discharge member 134 can be attached to the centrifugalpellet dryer 30 utilizing multiple fastener openings 141, in the samemanner as the fastener openings used to attach the base member 101 andthe base plate 128. As explained above, details of a manner ofassembling a pellet dryer are described in more detail in connectionwith FIGS. 13 and 14.

Referring now to FIG. 12, an embodiment of a rotor is 89 illustratedwhich, as shown, can have different lifter 92 configurations fordifferent regions of the rotor 144, 147, 150, 153. Generally, the rotor89 can have a greater concentration of lifters 92 provided in region147. The greater concentration of lifters can aid in providing thedesired increase in the number of impacts between lifters and pellets.The lifters 92 are positioned to “fill gaps” between lifters 92 in upperor lower adjacent rows. In other words, the lifters 92 are not alignedwith lifters 92 in adjacent rows whereby there is no unobstructed “path”between rows of lifters: the top edge of a lifters 92 in one row isvertically aligned with the bottom edge of a lifter 92 in the rowimmediately above. Vertical spacing was determined through simulation.Matching vertical blade gap to pellet velocity and blade speed canprovide the greatest transfer of pellets from the front of one lifter tothe back of the lifter in a row directly above. Reducing the verticalblade gap induced a spiral effect, increasing the vertical speed of thepellets, reducing the drying time and number of impacts. Increasing thevertical blade gap would cause the pellets to miss the back of the bladeabove. This resulted in losing control of pellet path, reducing thedrying time and number of impacts.

Although the rotor 89 is shown as a preferred type of rotor having rotormanifold 107 with hollow interior portion 110 and radially extendingpassages 113, it is to be understood that the invention is not limitedto this preferred type of rotor. In particular, any conventionalcentrifugal pellet dryer rotor could be used wherein lifters accordingto the invention are affixed to the rotor outer surface in a patternaccording to the invention to obtain the results described herein.

A first, lower-most region 144 of the rotor 89 have a single row oflifters which can generally be longer than the lifters in other regionsof the rotor 89. These lifters 157, which are generally adjacent slurryoutput from the radial passages 113 in the rotor manifold 107, can belonger to initially contact the slurry of pellets and water as it firstenters the gap 116 between the rotor outer surface 95 and the dewateringscreen 86. The next region 147 of the rotor 89 can be configured withstandard sized lifters 160, but with more rows of lifters 160 and alarger number of lifters 160 in each row. A third region 150 of therotor 89 can generally be a longer portion of the rotor 89 having morerows of lifters. However, there can be a fewer number of lifters 160 ineach row, and the spacing between lifters can thus be greater. A fourthregion of the rotor 89, generally adjacent the pellet discharge port 48,can have a single row of flat vertical exit blades 163 to facilitatedischarging the dried pellets out the pellet discharge port 48. The exitblades 163 and the discharge chamber are designed to change thedirection and focus the upward flow of pellets tangentially out thepellet discharge port 48.

According to the invention, a larger number of lifters and rows oflifters are provided on the surface 95 of the rotor 89 in the second 147and third regions 150 to provide a more efficient drying of the pellets.The majority of the water is shed from the slurry as the first row oflifters 157 sweep the entering slurry upwards. However, a large part ofthe dewatering process occurs as the pellets progress upwards throughthe second region 147. In the second region 147, the lifters 160 aremore numerous and are specially contoured to direct the pellets not onlyupwards toward the pellet discharge port 48, but also inwards toward thesurface 95 of the rotor 89, and the next row of lifters 160. Directingthe pellets inwards reduces the number of impacts between the pelletsand the dewatering screens 80, 83, 86, which enhances the dryingprocess. Drying can be degraded by increased impacts between the pelletsand the dewatering screens 80, 83, 86 because droplets of water that areretained on the dewatering screens can be reacquired by the pelletsthrough repeated impacts with the dewatering screens. Thus, limitingpellet impacts with the dewatering screens 80, 83, 86 can enhance thedrying process. Another problem with screen impacts is that the screencan act as a “grater” which shaves, or breaks off pieces of the pellets.These pieces of pellets, called “fines,” can negatively effect theprocess in a number of ways, for example by contaminating the dischargedwater and by “gumming” up the dewatering screens, bearings, and otherparts of the pellet dryer 30.

The third region 150 of the rotor 89 can require fewer lifters 160because most of the dewatering has been accomplished in the first 144and second 147 regions. The main factor effecting the number of lifters160 in the third region 150 is providing sufficient time to vaporizeremaining water by maintenance of the controlled upward movement of thepellets toward and out through the pellet discharge port 48.Accordingly, only the number of lifters 160 required to maintain aslower controlled upward flow of the pellets need be provided on therotor surface 95 in the third region 150. Some of the drying effected inthe second 147 and third 150 regions can be the result of internal heatof the pellets. Since the pellets may often be delivered in a slurry tothe centrifugal pellet dryer 30 directly after the pelletizationprocess, the pellets can typically still contain some internal heat. Inthe second 147 and third 150 regions of the centrifugal pellet dryer 30,limiting screen contacts, and thus limiting contact with water dropletson the dewatering screens 80, 83, 86, permits the internal heat of thepellets to continue the drying process while the pellets are carriedupwards. In a presently preferred embodiment, 8 lifters 160 can beutilized in each row in the second region 147 whereas half as manylifters 160 can be used in each row in the third region 150. However,the number of rows in each region 144, 147, 150, 153, and the number oflifters in each row, can be dependent upon the length and diameter ofthe rotor 89.

It should also be understood that the “regions” 144, 147, 150, 153 ofthe rotor 89 are not necessarily coextensive with each section 63, 66,69 of the pellet dryer 30—which are generally defined by the separatorplates 71, 74, 77. In particular, regions of different lifterconfigurations on the rotor 89 need not be confined to any singlesection 63, 66, 69 of the pellet dryer 30 or any single dewateringscreen 80, 83, 86. In other words, any particular region 144, 147, 150,153 of the rotor 89 may extend through adjacent sections 63, 66, 69and/or dewatering screens 80, 83, 86.

Regarding assembly of a preferred embodiment of the pellet dryer 30,FIGS. 13 and 14 illustrate internal frame members 169, 172 having anumber of vertically extending support members 175, 178, four in theseembodiments, which can provide the primary structural framework for thecentrifugal pellet dryer 30. As shown, separator plates 181-194 can beutilized at various positions along the vertical supports 175, 178 todivide the pellet dryer 30 into different sections. These sections cangenerally separate the centrifugal pellet dryer 30 into a number ofdistinct chambers, such as the three sections 198-204 shown in FIG. 13,or the two sections 206, 209 shown in FIG. 14. The separator plates canbe provided with openings 212 which can permit a vacuum created viaexhaust port 51 to be applied to each chamberized section 198-204 toimprove the drying process, as explained previously in connection withthe description of FIG. 3. However, more than three sections, or asingle section, could also be utilized according to the invention. Thevertical support members 175, 178 can be a single cylindrical member ormay be assembled from sections which fasten together, such as at thejunctions of the separator plates 181-194 and the vertical supports. Thepellet dryer 30 can be assembled by connecting the base member 101, withthe base plate 128 thereon, to the bottom of the vertical supportmembers 175, 178 via mating fastener openings provided in the basemember 101, the base plate 128, and in each end of each vertical support175, 175, through which an appropriate fastener can be disposed to jointhe components together. Next, the rotor 89, with the rotor manifold 107connected thereto, can be disposed down through the center openingformed in each separator plate such that the lower portion 119 of therotor manifold 107 is connected within the receptacle 122 formed in thebase member 101. Of course, the water bearing 318 and seal 324 can beinstalled earlier if appropriate. The pellet discharge member 134 can beconnected over the top end of the rotor 89 with the rotor shaft member98 disposed through an opening 139 provided in the pellet dischargemember 134 for connecting the rotor shaft member 98 to the output shaftof the motor 57, which can be connected atop the pellet discharge member134. The pellet discharge member 134 can be connected to the verticalsupport members via mating fastener openings provided therein to connectto each vertical support 175, 178. The dewatering screens 80-86 can beremovably positioned in each section 63-69, or 198-209, around the rotor89 and secured in place. Finally, the outer cover, or housing 54, can beremovably positioned over the entire framework 169, 172 and secured inplace. To operate the pellet dryer 30, a source of slurry can beconnected to the slurry inlet 42 and the motor 57 operated to rotate therotor 89 to begin pumping the slurry through the pellet dryer 30.

A presently preferred embodiment of a lifter 92 which can be affixed tothe surface 95 of the rotor 89, or, it is to be understood, to any priorart rotors for conventional centrifugal pellet dryers, is illustrated inFIGS. 15 through 18. Typically, the lifter 92 is affixed to thecylindrical rotor surface 95 at an upwardly inclined angle of 45degrees, thus defining a typical helix curvature. Additionally, asexplained previously, the lifter 92 can preferably be configured todeflect the pellets inwardly toward the surface 95 of the rotor 89 inorder to limit the number of impacts between the pellets and thedewatering screen 80, 83, 86. A presently preferred configuration forthe lifter 92 is to provide an inwardly curved surface on the front faceof the lifter, wherein the front face is facing the direction ofrotation of the rotor 89. The inwardly curved surface tends to controlthe pellet path, keeping the pellets in the lifter envelope and awayfrom the dewatering screens. The arrangement of the lifters on therotor, generally a higher concentration of lifters on the lower regionof the rotor, works in combination with the inwardly curved surface ofthe lifters 92 to provide increased pellet impacts with the lifters.When a pellet hits a lifter 92, water is transferred to the lifter 92and centrifugal force works that water out to the dewatering screens 80,83, 86. Thus, the lifter 92 is generally continuously cleaning itself ofwater. However, although much of the water passes through the dewateringscreens 80, 83, 86, some of the water is retained on the inside surfaceof the screens 80, 83, 86 where it can be reacquired by pelletsimpacting the screen. Pellet impacts with the dewatering screens aretherefore generally undesirable—hence advantage of inwardly contouredlifters 92. Consequently, it is clear that it is the inwardly curvingshape of the lifter, and/or in the configuration of lifters, e.g., moreor fewer lifters in each row in different regions along the length ofthe rotor, which results in much of the advantages obtained according tothe invention.

Generally, the contour of a presently preferred embodiment of the lifter92, which generally corresponds to lifter 160 in FIG. 12, can best bedescribed as being characterized by the top and bottom edge of thelifter 92 defining an involute curve. The lifters 144 in FIG. 12 canhave the identical curvature, except that the lifters 144 arelonger—twice as long in a preferred embodiment. It should also beunderstood that the lifters are attached to the rotor in a conventional45 degree helix configuration. The 45 degree helix configurationprovides the lift to carry the pellets upwards through the pellet dryer30. However, it is the involute curvature, i.e., the inwardly curvingaspect, of the lifter 92 which can reduce the number of screen impacts.The exact contour of such a lifter 92 can be dependent on the diameterof the rotor 89. For example, a presently preferred lifter can have acurvature defined by the following equations:

Xcoord=Inner Radius (COS Degree+Radian×SIN Degree)

Xcoord=Inner Radius (SIN Degree+Radian×COS Degree)

The following table contains values for Xcoord and Ycoord which could beutilized to create a presently preferred lifter having an involutecurvature defined by those equations.

Degree Radian Inner Xcoor Ycoord Outer/Inner  1 0.017 4.000 4.001 0.0001.71875  3 0.052 4.005 0.000  5 0.087 4.015 0.001  7 0.122 4.030 0.002 9 0.157 4.049 0.005 11 0.192 4.073 0.009 13 0.227 4.102 0.015 15 0.2624.135 0.024 17 0.297 4.172 0.035 19 0.332 4.214 0.048 21 0.366 4.2600.065 23 0.401 4.309 0.085 25 0.436 4.363 0.109 27 0.471 4.420 0.136 290.506 4.480 0.168 31 0.541 4.543 0.205 33 0.576 4.609 0.246 35 0.6114.678 0.293 37 0.646 4.749 0.344 39 0.681 4.822 0.401 41 0.715 4.8960.464 43 0.750 4.972 0.532 45 0.785 5.050 0.607 47 0.820 5.127 0.687 490.855 5.206 0.774 51 0.890 5.284 0.867 53 0.925 5.362 0.967 55 0.9605.439 1.074 57 0.995 5.516 1.187 59 1.030 5.590 1.307 61 1.064 5.6631.433 63 1.099 5.734 1.566 65 1.134 5.803 1.707 67 1.169 5.868 1.853 691.204 5.930 2.007 71 1.239 5.989 2.167 73 1.274 6.043 2.334 75 1.3096.093 2.507 77 1.344 6.137 2.687 79 1.379 6.177 2.873 81 1.413 6.2113.065 83 1.448 6.239 3.262 85 1.483 6.260 3.466 87 1.518 6.275 3.675 891.553 6.282 3.889 91 1.588 6.282 4.108

Although a preferred embodiment of the lifter 92 comprises providing aninvolute curvature, such as according to the foregoing equations, itwill be appreciated by those skilled in the art that various otherconfigurations of the lifter 92 could be devised which also deflectpellets inwardly toward the rotor surface away from the dewateringscreens. For example, other equations which define an inwardly curvingsurface could be formulated. Also, the lifter 92 surface could be formedfrom a series of flat surfaces bent or joined at angles to each otherdefine a surface which depends inwardly toward the rotor surface.Additionally, a less sophisticated configuration could be to simplyprovide a flanged, or bent, portion at the outermost edge of the lifter92. The flanged portion could be angled toward the direction of rotationof the rotor such that pellets striking the flanged portion would bedeflected toward the surface of the rotor and other lifters.

The vertical spacing between rows of lifters 92 can preferably be set toeliminate any horizontal gaps between the rows. The number of lifters 92in each row can be dependent upon diameter of the rotor 89 and the speedof rotation. The spacing between each lifter 92 in rows in, for example,the second region 147 in FIG. 12, is set to minimize pellet slippage andprovide a maximum amount of impacts between pellets and the lifters 92.For example, in an embodiment of a rotor having an 8 inch diameter androtated at about 1200 rpm, 8 lifters can be used in each row. In thethird region 150, for example, resonance only requires a minimum numberof impacts to keep the pellets moving upwardly and to provide time forinternal heat of the pellets to vaporize any remaining film of water onthe surface of the pellets. In the example embodiment described aboveusing 8 lifters in each row in the second region 147, only 4 lifters ineach row of lifters in the third region 150 can be needed to satisfy therequirements described above. The number of rows of lifters in eachregion can be largely dependent upon the length and diameter of therotor 89. Generally, the lifters 92 are contoured and configured tocreate, as an example, a pellet path described as follows:

A pellet hits the front of one lifter which accelerates and lifts thepellet to the next row of lifters. The pellet hits the back of thelifter in the row directly above, which decelerates and stops the liftof the pellet. The pellet then hits the front of a following lifter inthe same row, which accelerates and lifts the pellet up to the next rowof lifters. The process repeats until the pellet reaches the pelletdischarge port.

Consequently, the configuration of the front surface of the lifter 92and the arrangement of the lifters 92 on the rotor 89 permits somedegree of control over the pellet path to optimize and control theprocess. For example, the top of one row of lifters 92 can be verticallyaligned with the bottom of an adjacent row of lifters 92. Verticalspacing has been determined, such as by using computer simulation, tomatch vertical gap between rows of lifters 92 to pellet velocity andlifter speed (rotational speed of the rotor) to provide the greatesttransfer of pellets from the front of a lifter 92 in one row to the backof a lifter 92 in the row directly above. Reducing the vertical gap,from an optimal distance, induces a spiral effect, which can undesirablyincrease the vertical speed of the pellets, thus reducing the availabledrying time and number of impacts between the pellets and the lifters.Additionally, providing too large a gap can result in the pelletsmissing the back of lifters in an upper adjacent row, which candisadvantageously result in a loss of control over the pellet path, alsoreducing impacts between pellets and lifters and reducing the availabledrying time.

A presently preferred rotor diameter can be 8 inches, but could besmaller or much larger, such as 36 inches. The height of each lifter 92,and the spacing therebetween, can be dependent on the rotational speedat which the rotor 89 is operated. A presently preferred speed is astandard motor speed of 1150-1200 rpm, although a range of about 700 to1800 rpms can also be utilized. Other factors in the system include thediameter of the rotor 89 at the outer tip of the lifters 92. For an 8inch diameter rotor 89, a presently preferred diameter at the outer tipof the lifters 92 can be 13.75 inches. The surrounding dewateringscreens, 80, 83, 86 can have a presently preferred diameter of 14inches. However, variations of these dimensions can be providedsatisfactorily employed according to the invention. In general, computersimulation was used in the development of the invention to optimizecertain elements of the lifter contour and configuration for variousrotor diameters, lengths, rotational speeds, and other factors. Thus,pellet dryers having different operational requirements can be designedin accordance with the invention but with customized elements, such aslifter contour, configuration of lifters on the rotor, rotor diameter,length and rotational speed, number of chamberized sections of thepellet dryer, and the like.

Referring to FIGS. 19 and 20A-20H, a manner of providing a presentlypreferred configuration of lifters or the rotor 89 is illustratedwherein attachment points on the rotor 89 for each lifter are provided.The layouts in FIGS. 19 and 20A-20H to provide a pattern for a presentlypreferred configuration of lifters on a rotor having an 8 inch outsidediameter. The number of rows of holes “F” and “G: can vary dependingupon the length of the rotor 89. For example, a pellet dryer 30 with 3sections, as shown in FIG. 3, could have a 8 inch diameter rotor 89 withan overall length of 59 inches and 26 rows of holes “F” and “G.”Alternatively, a four screen model could have an overall length greaterthan seven feet and more than 30 rows of holes “F” and “G.” It should beobserved that the pairs of attachment holes, reference number 218 forexample, must be space and positioned to correspond to the attachmentpins 214, 215. provided on the lifter 92. In a preferred embodiment, thedistance “w” between each pair of attachment holes can be the same, andcan be 2 inches.

In FIGS. 20A, 20B, 20D and 20E, the pattern is for attaching the longerlifters, i.e., lifters 157 in FIG. 12. Four lifters are provided equallyspaced around the diameter. FIG. 20C indicates that four openingsequally spaced around the diameter are provided for the radiallyextending passages 113 through which the slurry is conducted from therotor manifold 107 into the pellet dryer 30. As shown in FIG. 12,preferably 8 lifters 92 can be provided in region 147 of the rotor 89,i.e., the first 8 rows of lifters 92. Thereafter, only 4 lifters 92 canbe provided in region 150 of the rotor 89, e.g., each of the 18subsequent rows of lifters. As described above, a presently preferred3-screen embodiment of the rotor 89 can have 26 rows of holes “F” and“G,” for attaching the 8 rows of 8 lifters 92 and subsequent 18 rows of4 lifters 92.

Further aspects of a preferred embodiment of the invention areillustrated in FIGS. 21 through 23 which show additional details of thelower end to the pellet dryer 30. As shown, the rotor manifold 107 issupported by a bearing 318 which runs in ordinary water. The waterbearing 318, shown best in FIG. 22, can be “cooled” using fresh make upwater provided via a water inlet 321. Use of a water bearing 318 avoidsa common problem with conventional pellet dryers wherein water from theslurry migrates into the rotor bearings thus ruining the bearing andrequiring premature replacement. In the present case the bearing 318 isdesigned to run in water so premature failure due to water contaminationcan be entirely eliminated. Additionally, a seal 324 can be used at theinterface between the slurry inlet 42 and the rotor manifold 107. Asshown in FIG. 21, the slurry inlet 42 creates a water trap 327 which,along with the seal 324, seals the slurry inlet 42. FIG. 23 illustratesa water containment member 329 attachable to the base member 101 whichcollects water 330 discharged through water discharge outlets 45 andsubsequently diverts the discharged water 330 through drain 336. Thewater containment member 329 can have a wall portion 342 between thedrain 336 and the water discharge outlets 45. The wall portion 342 actslike a dam which and helps to maintain the water 330 in the watercontainment member 329 at a level just above the opening of the waterdischarge outlets 45 such that a water seal is created. The water trap327 created by the slurry inlet 42 and the water seal created by thewater containment member 329 work to prevent air from entering into thelower dewatering regions 144, 147 and moisture from rising up throughthe pellet dryer 30 as the pellets are carried upwards toward the pelletdischarge outlet 48. These features can enhance the advantages obtainedusing the application of a vacuum via the exhaust port 51, as previouslydescribed herein.

Although certain embodiment of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications to those details could be developed in light of theoverall teaching of the disclosure. Accordingly, the particularembodiments disclosed herein are intended to be illustrative only andnot limiting to the scope of the invention which should be awarded thefull breadth of the following claims and any and all embodimentsthereof.

What is claimed is:
 1. A lifter for a centrifugal pellet dryer rotorcomprising a blade attachable to the surface of said rotor and saidblade having a front surface configured to deflect said pelletsgenerally inwardly toward said outer surface of said rotor, said frontsurface facing a direction in which said rotor is rotated.
 2. The lifterof claim 1 further comprising said front surface having upper and loweredges, and said upper and lower edges defining an involute curve.
 3. Thelifter of claim 2 wherein said upper and lower edges have a curvaturedefined by the following equations: Xcoord=Inner Radius (COSDegree+Radian×SIN Degree); and Ycoord=Inner Radius (SINDegree+Radian×COS Degree).